There are various instruments and tools used for verifying the horizontality or the verticality of a surface like, for example, levels and pendulums. Levels allow in particular to draw and build levelled surfaces, generally horizontal or vertical, and sometimes at a predetermined angle like at 45°. Levels may be used to draw a marking or reference line going from one point to another while being well parallel with the sea level (horizontal) or at 90° from it (vertical). Levels are used for the construction of walls and floors so that they are properly oriented. A plumb bob hanging from a wire is normally used as a pendulum to verify the verticality.
The principle of a pendulum is that of the plumb-line. The line is fixed at one of its ends and passes at the other end before a reading table fixed to a work (dam, building, etc.). Depending on whether the attachment point of the wire is located upward or downward, it is called a direct pendulum or an inverted pendulum. For the direct pendulum, the tensioning of the wire is ensured by a weight whereas it is ensured by a floater for an inverted pendulum. The reading table, with aiming points, is used for the precision. The pendulum exhibits qualities similar to those of auscultation apparatuses where frequent readings can be performed.
To sum up, the use of a level or a pendulum is a simple and efficient solution to level a non-immerged surface. However, for immerged or partially immerged works, an inverted pendulum needs to be considered. Unfortunately, the inverted pendulum quickly looses its precision if the water is not completely stagnant. To make it sturdier, the tension in the wire must be increased, which involves increasing the size of the floater, so that it becomes cumbersome and heavy, to the extent that it becomes practically impossible to have several wires brought closer at short distance. However, such a need exists, for example, to achieve a levelling used for the inspection of a partially or fully immerged structure like a dam.